C8 alkylaromatic hydrocarbons are generally considered to be valuable products, with the highest demand being for para-xylene. Major sources of para-xylene include mixed xylene streams that result from the refining of crude oil. Examples of such streams are those resulting from commercial xylene isomerization processes or from the separation of C8 alkylaromatic hydrocarbon fractions derived from a catalytic reformate by liquid-liquid extraction and/or fractional distillation.
A simulated moving bed (“SMB”) adsorption process is used commercially in a number of large scale petrochemical separations to recover high purity para-xylene from mixed xylenes. As used herein, the term “mixed xylenes” refers to a mixture of C8 aromatic isomers that includes ethylbenzene, para-xylene, meta-xylene and ortho-xylene. High purity para-xylene may be used for the production of polyester fibers, resins and films by converting para-xylene to terephthalic acid or dimethyl terephthalate, which is then reacted with ethylene glycol to form polyethylene terephthalate, and the raw material for most polyesters.
The general technique employed in the performance of SMB adsorptive separation processes is widely described and practiced. Generally, the process simulates a moving bed of adsorbent with continuous counter-current flow of a liquid feed over the adsorbent. Feed and products enter and leave adsorbent beds continuously, at nearly constant compositions. Separation is accomplished by exploiting the differences in affinity of the adsorbent for para-xylene relative to the other C8 aromatic isomers. More specifically, the adsorbent is selected for its initial affinity for para-xylene relative to the other C8 aromatic isomers.
In order to desorb the para-xylene, a desorbent is used for which the adsorbent has a higher affinity relative to para-xylene. One such desorbent is para-diethylbenzene, a heavy desorbent, and which is two carbon numbers heavier than the xylenes. It is also known to use toluene, a light desorbent, which is only one carbon number lighter than the xylene feed. As a desorbent, toluene is comparatively weaker than para-diethylbenzene. Since the toluene is weaker, it requires a higher desorbent to feed ratio (D/F) and more energy for separation in extract and raffinate fractional distillation columns.
A typical SMB adsorptive process for producing para-xylene produces a single extract stream enriched in para-xylene and a single raffinate stream containing the remaining ethylbenzene, meta-xylene, and ortho-xylene from the feed. It is also possible to take two raffinate streams from the process as disclosed in U.S. 20150087878A1 and U.S. 20150266794A1 resulting in one raffinate which is enriched in ethylbenzene and the other which is enriched in meta-xylene and ortho-xylene. These two streams preferentially enriched or depleted in ethylbenzene can then be processed downstream separately, such as in separate xylene isomerization units, to take advantage of the C8 aromatic compositional differences.
More recently, U.S. Pat. No. 9,850,186B2 has shown that the two raffinates can be taken such that differences in both the C8 aromatic and toluene desorbent compositions are observed. These two raffinates are then processed separately with the desorbent-lean raffinate passing directly to a separate xylene isomerization unit from the desorbent-rich raffinate stream. While possible, this configuration requires the construction of two separate xylene isomerization units and use of toluene as a desorbent as well as the enrichment of different C8 aromatic species to take full advantage of the configuration.
FIGS. 1A and 1B illustrate separation processes using a single raffinate stream with light and heavy desorbents. FIG. 1A shows an adsorption separation process 10 using a single raffinate stream and a heavy desorbent. The C8 aromatics feed stream 15 is sent to the adsorption separation unit 20. A para-xylene extract stream 25 is recovered from the adsorption separation unit 20. A raffinate stream 30 comprising desorbent and the other C8 aromatics (ethylbenzene, meta-xylene and ortho-xylene) is sent to fractional distillation column 35 where it is separated into an overhead stream 40 comprising the other C8 aromatics and a bottom stream 45 comprising the desorbent. The bottom stream 45 is recycled to the adsorption separation unit 20. The overhead stream 40 is sent to an isomerization unit 50 where the C8 aromatics are isomerized to produce additional para-xylene.
In FIG. 1B, a light desorbent is used in process 75. In this case, the raffinate stream 30 is separated into an overhead stream 55 comprising the desorbent and a bottom stream 60 comprising the other C8 aromatics. The bottom stream 60 is sent to the isomerization unit 65.
Because fractionation is one of the largest energy users in an aromatics complex, it would be desirable to reduce the fractionation costs.
Therefore, there is a need for an improved process for producing p-xylene having reduced operating costs.